Conventionally, as a method of removing microbial particles from a solution, a gel filtration method, a centrifugation method, an adsorption separation method, a precipitation method, a membrane filtration method, etc. are utilized. However, the gel filtration method is difficult to be industrially applied due to the facts that a target substance is diluted with the solvent used in gel filtration, the method is not suitable for large-scale processing, etc. The centrifugation method can only be applied in the case where the size of a microbial particle is not smaller than a few μm and the solution viscosity is low. The adsorption filtration method can be utilized for removal of a small amount of certain microbial particles, however, this method cannot be applied to a solution in which a variety of microorganisms are dispersed in a large amount. The precipitation method can be utilized in processing of a relatively large amount of solution, however, this method is not capable of completely removing microbial particles by itself.
On the other hand, the membrane filtration method utilizing a micro-filtration membrane or an ultrafiltration membrane is capable of removing any microorganism and, in addition, is capable of large-scale successive processing, thereby being suitable for industrial utilization.
However, as for conventional micro-filtration membranes and ultrafiltration membranes, there is a problem that a concentrated layer is formed from microorganisms removed on the surface of the membrane or the disrupted matters thereof and the surface of the membrane is blocked, so that an increase in filtration pressure and an over time decrease in filtration rate are likely to be caused.
To remove such a concentrated layer on the surface of the membrane, backwash may be performed by introducing a fluid opposite in the direction of filtration, however, a flow of a large amount of backwash fluid is required to wash off materials strongly adsorbing to the surface of the membrane, thereby filtration efficiency decreases.
In Patent Document 1, a hollow fiber membrane having a gradient structure with progressively decreasing pore sizes from the membrane inner surface toward the membrane outer surface and composed of polysulfone and polyvinylpyrrolidone is described. To obtain this hollow fiber membrane, polyvinylpyrrolidone of a relatively low molecular weight is used. In addition, removal of polyvinylpyrrolidone in the membrane on purpose from the hollow fiber membrane after membrane formation is described.
In Patent Document 2, a hollow fiber membrane which has a spongy structure throughout the membrane and has a dense layer with a thickness of 0.05 to 0.3 μm on at least one of the inner surface side and the outer surface side is described.
In addition, as cross-flow filtration or a filtration method utilizing countercurrent washing (backwash), for example, in Patent Document 3, a method of removing components that cause turbidity by performing backflushing for a short period of time is described. Furthermore, in Patent Document 4, a method of removing turbidity in which dead-end filtration and backwash for a short period of time are combined is described.